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Does gold come from outer space?

By William Kremer

BBC World Service

The idea that gold came from outer space sounds like science fiction,
but it has become well-established - it's pretty much received opinion
in the field of earth sciences. How did this bizarre theory take hold,
and is it here to stay?
For the chieftains of pre-Columbian America, the dazzling yellow
stuff they found glinting at the bottom of streams or buried in the
rocky ground captured the power of the sun god. They dressed themselves
in battle armour wrought from the enchanted metal, believing it would
protect them.
They were sadly deceived.
Gold, an unusually soft metal, wasn't any match for the steel of the
Spanish. But the Native Americans may well have been right in believing
the element was otherworldly.
"Why do you find nuggets of gold on the surface of the Earth?" asks
science writer John Emsley. "The answer to that, is that they've arrived
here from space in the form of meteorites."
This theory has come in the last few decades to be held by the
majority of scientists as a way of explaining gold's abundance. There
may only be 1.3 grams of gold per 1,000 tonnes of other material in the
Earth's crust (the rocky shell of the planet that is around 25 miles
thick) but that's still too much to fit with the standard models of our
planet's formation.
After its birth four-and-a-half billion years ago, the surface of the
Earth heaved with volcanoes and molten rock. Then, over tens of millions
of years, most of the iron sank down through the outer layer, known as
the mantle, to the Earth's core. Gold would have mixed with the iron and
sunk with it. Matthias Willbold, a geologist at Imperial College London,
likens the process to droplets of vinegar collecting at the bottom of a
dish of olive oil.
"All the gold should be gone," he says.
It isn't though. So science has had to come up with an explanation,
and the answer currently favoured is - a meteoric shower.
"The theory is that after the core formed there was a meteoric shower
that struck the Earth," says Willbold. "These meteorites contained a
certain amount of gold and that replenished the Earth's mantle and the
continental crust with gold."
Willbold says the theory fits with the pattern of meteorite activity
as scientists understand it, climaxing with a huge storm that took place
more than 3.8 billion years ago, referred to as the "terminal
bombardment". The meteorites punched out the craters we see on the moon
and came from an asteroid belt that still exists between Earth and Mars.
This idea of the gold-laden-meteorite "veneer" was first proposed
following the Apollo moon landings of the 1970s. Scientists examining
rock samples from the moon's mantle found much less iridium and gold
than they did in samples from the surface of the moon or from the
earth's crust and mantle. It was proposed that the moon and Earth had
been battered by iridium-rich meteorites, known as chondrites, from
outer space. While the precious fallout from this meteoric shower lay
scattered on the surface of the moon, on Earth the planet's internal
activity had churned it into the mantle too.
The idea, called the "late veneer hypothesis", has become a
fundamental theory in planetary science.
It also helps to explain many other anomalies in the Earth's
composition - it is thought that the same meteorites delivered the
carbon, nitrogen, water and the amino acids that are vital to all life
on the planet.
"They are basically the building blocks of Earth," says Willbold.
Two years ago, he and a team from the universities of Bristol and
Oxford examined some rocks from Greenland which had their origins in a
part of the Earth's mantle that was insulated from meteorite activity
for a crucial period some 600 million years. The team did not look at
the gold content of the 4.4-billion-year-old rocks, but at tungsten.
Tungsten has some similarities to gold but exists in different forms or
isotopes, and this provides scientists with more historical information.
"The tungsten-isotopic composition of these rocks was basically
really different from the tungsten-isotopic composition of other rocks,"
says Willbold.
He infers that the Greenland rocks are a remnant of Earth's
composition prior to the start of the late veneer meteorite shower,
postulated to have taken place between 4.4 and 3.8 billion years ago.
Willbold's influential study, published in Nature in September 2011,
provides the most compelling evidence yet for the late veneer
hypothesis. This hypothesis seems the best explanation for the unusual
tungsten-isotopic profile of Willbold's Greenland rocks, just as it
seemed to explain the different quantities of gold and iridium in the
mantles of the Earth and the Moon in the 1970s.
But the hypothesis has been challenged.
Last year, Mathieu Touboul and a team from the University of Maryland
examined some different rocks, this time from Russia and significantly
younger than those in the Greenland study - a mere 2.8 billion years
old.
These younger rocks had their full complement of elements known as
siderophiles - the iron-loving group of metals that includes gold - but
in terms of tungsten isotopes, the rocks turned out to be very similar
to Willbold's. And yet they date from after the time proposed for the
late veneer bombardment.
"We reach a different conclusion about what is generating these
tungsten anomalies inside the rocks," says Touboul. He thinks
differences in the Earth's mantle might have caused tungsten isotopes to
develop in different ways.
Touboul though still believes the late veneer hypothesis is right -
he just doesn't think that tungsten isotope measurements provide a
demonstration of it.
Other scientists think it's time for a major rethink.
"I used to accept the late veneer hypothesis back when we had so
little data that it seemed to be a sensible interpretation, but I think
it's past its prime now," says Munir Humayun of Florida State
University.
"It seemed so elegant, but there were so many gaps in the data. We
presumed a lot and knew very little back then."
Humayun says the original 1970s studies on moon and Earth rocks
produced imprecise results, at variance with more sophisticated
follow-up studies from the 1990s.
One of these studies, from the University of Maryland, found less
resemblance than expected between the Earth's rocks and chondrites - the
iridium-rich meteorites. "This is where the late veneer failed in my
opinion," says Humayun. "The answer came back that none of the known
meteorite types were anything like the veneer."
Scientists also began to find metals like gold much deeper in the
Earth's mantle than they had anticipated. This could be explicable if
the Earth underwent a much bigger meteoric barrage than originally
supposed, and at an earlier point in time. But the way Humayun sees it,
the late veneer hypothesis stopped answering old questions - and started
posing new ones.
He is one of a small group of scientists who subscribe to an
alternative theory. Their proposition is that all the gold in the
Earth's crust - or the overwhelming majority of it - was here on Earth
all along. Most of it certainly alloyed with iron and migrated to the
Earth's core, but a significant proportion - perhaps 0.2% - dissolved
into a 700km deep magma "ocean" within the Earth's outer mantle.
Later, the gold was brought back up to the crust by volcanic action.
This is the stuff we wear round our necks and on our fingers today.
This theory requires gold and other siderophile elements to be more
soluble than has previously been thought, otherwise insufficient
quantities would have dissolved in the magma.
Experiments by two scientists at Nasa - Kevin Righter and Lisa
Danielson - indicate that gold's solubility in mantle rocks does
increase with high pressures and temperatures.
However, it has not yet been possible to measure in a lab the
solubility of all the highly siderophile elements over the full range of
temperatures and pressures of the Earth's mantle, so for now this
proposed explanation for the abundance of gold also remains no more than
a hypothesis. But it is attracting interest and was bashed against the
late veneer theory at length in a session last month at geochemistry's
annual international symposium - the Goldschmidt Conference in Florence.
Matthias Willbold, who attended the session, says the consensus in
the room was that the late veneer hypothesis was still the best
explanation for the unusual tungsten-isotopic profile of his Greenland
rocks.
He adds that, unlike Humayun, most scientists believe that chondritic
meteorites are a "match" for concentrations of metals in the Earth's
mantle and crust. But he says he accepts that the case for the late
veneer hypothesis is not exactly sewn-up.
"You can never be absolutely sure," he says. "But the beauty of our
model at the moment is that all the numbers match up very well." His
isotope measurements indicate that about 0.5% of the Earth's mantle mass
fell in the form of meteorites (that's 20 billion billion tonnes, if you
were wondering). This figure matches geologists' current best guess,
based on the overall concentrations of precious metals in the Earth's
mantle and crust. Willbold describes this match as a "smoking gun".
But Humayun says that the extent to which geochemists believe it
depends on their precise field of study.
Analytical geochemists - the group of researchers that measures trace
elements in rocks - have come to see their research as crucial to
understanding the emergence of life on Earth. Humayun says that
experimental geochemists - the group of scientists attempting to
recreate the conditions of the mantle in the lab - are more open-minded.
"It's about how you make your money! If you're an experimentalist,
then you're eating the late veneer guys' lunch by doing these
experiments.
"Why the analytical community likes the idea (of a late veneer) so
much is something that continues to trouble me. It's because of this
relevance they have tied in to the origin of life. There's a lot riding
on it!"

http://www.bbc.co.uk/news/magazine-22904141
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Bye, all! Alexander Koryagin
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